Disclosure to Promote the Right To Information

Whereas the Parliament of India has set out to provide a practical regime of right to information for citizens to secure access to information under the control of public authorities, in order to promote transparency and accountability in the working of every public authority, and whereas the attached publication of the Bureau of Indian Standards is of particular interest to the public, particularly disadvantaged communities and those engaged in the pursuit of education and knowledge, the attached public safety standard is made available to promote the timely dissemination of this information in an accurate manner to the public.

इंटरनेट मानक

“!ान $ एक न' भारत का +नम-ण”Satyanarayan Gangaram Pitroda

“Invent a New India Using Knowledge”

“प0रा1 को छोड न' 5 तरफ”Jawaharlal Nehru

“Step Out From the Old to the New”

“जान1 का अ+धकार, जी1 का अ+धकार”Mazdoor Kisan Shakti Sangathan

“The Right to Information, The Right to Live”

“!ान एक ऐसा खजाना > जो कभी च0राया नहB जा सकता है”Bhartṛhari—Nītiśatakam

“Knowledge is such a treasure which cannot be stolen”

“Invent a New India Using Knowledge”

है”ह”ह

IS 15681 (2006): Geological exploration by geophysicalmethod (seismic refraction) - Code of practice [WRD 5:Gelogical Investigation and Subsurface Exploration]

IS 15681:2006

Indian Standard

GEOLOGICAL EXPLORATION BY GEOPHYSICALMETHOD (SEISMIC REFRACTION) —

CODE OF PRACTICE

ICS 93.020

(3 BIS 2006

BUREAU OF INDIAN STANDARDSMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG

NEW DELHI 110002

November 2006 Price Group 7

Geological Investigations and Subsurface Exploration Sectional Committee, WRD 05

FOREWORD

This Indian Standard was adopted by the Bureau of Indian Standards, after the draft finalized by the GeologicalInvestigations and Subsurface Exploration Sectional Committee had been approved by the Water Resources DivisionCouncil.

Seismic refraction technique is mainly concerned about evaluation of the project site and its stability to man-madestructures. The technique plays a crucial role in the design of engineering projects. Seismic refraction surveys,being rapid and economical, are conducted to help select a site amongst a number of alternatives at thereconnaissance stage. It also forms a part of detailed site investigations at the chosen location. It plays a majorrole in locating fault and shear zones and in determining engineering parameters (for example, Poisson’s ratio,dynamic Young’s and shear moduli of elasticity).

It is a reliable tool for determining depth to various subsurface layers, particularly in conjunction with a fewexploratory drillings. The accuracy of depth determination has been improved substantially with the availabilityof multichannel digital enhancement seismographs and new interpretation techniques, using digital computers.

This standard deals with various aspects of seismic reflection technique and its applications to shallow subsurfaceexploration of engineering sites. The primary purpose of the standard is to provide working knowledge of themethod, with relevant references, and with a basis to weigh the applicability of the method to various engineeringgeological problems. In particular, it seeks to provide an understanding of the proper planning of surveys, so as toobtain adequate and relevant coverage and highlight the most important area of interpretation of seismic data.

It has been assumed in the formulation of this standard that the execution of its provisions is entrusted toappropriately qualified and experienced people, for whose guidance it has been prepared.

IS 15681:2006

Indian Standard +

GEOLOGICAL EXPLORATION BY GEOPHYSICALMETHOD (SEISMIC REFRACTION) —

CODE OF PRACTICE

1 SCOPE

1.1 This standard deals with seismic refraction methodincluding the equipment, field procedures, andinterpretation of data for assessment of subsurfacematerials. Seismic refraction measurements, asdescribed in this standard, are applicable in mappingsubsurface conditions for various uses includinggeological, geotechnical, hydrological, environmental,mineral and archaeological investigations. Thecalculated seismic wave velocity is related to the typeof rock, degree of weathering, and rippabilityassessment on the basis of seismic velocity and othergeologic information.

1.2 Interpretation of the data collected during theseismic refraction survey is referred to in this standardonly in a general way. For full details of the theory,field procedure, or interpretation of the data, referenceshould be made to specialized texts.

2 TERMINOLOGY

For the purposes of this standard, the following termsand definitions shall apply.

2.1 Apparent Velocity — The velocity which a wavefront appears to have along a line of geophones. It isthe inverse of the slope of a refraction time-distancecurve.

2.2 Blind Zone — A layer which cannot be detectedby refraction methods, also called hidden layer. Theblind zone may have a velocity lower than that ofshallower refractors.

2.3 Compressional Wave (P-wave) — An elasticbody wave in which particle motion is in the directionof propagation; the type of seismic wave assumed inconventional seismic exploration. Also called P-waveand longitudinal wave.

2.4 Critical Distance — The offset at which arefracted event becomes the first break, the intersectionpoint for the travel time curves for two refractors.

2.5 Delay Time — The additional time taken for awave to follow a’trajectory, to and along, a buriedmarker over that which would have been taken to follow

the same marker, considered hypothetically to be, atthe ground surface or at a reference level.

2.6 Geophone — The instrument used to transform“seismic energy into an electrical voltage.

2.7 Geophone Interval — The distance betweenadjacent geophones within a group.

2.8 Head Wave — A refraction wave or Mintrop wave;a wave characterized by entering and leaving the highvelocity medium at critical angle.

2.9 Huygen’s Principle — Theconcept that every pointon an advancing wavefront can be regarded as the sourceof a secondary wave and that a later wavefront is theenvelope tangent to all secondary waves.

2.10 Hydrophore — A pr~ssure detector which issensitive to variations in pressure. Used when thedetector can be placed below a few feet of water as inmarine or marsh work, or as a well seismometer.

2.11 Intercept Time — The time obtained byextrapolating the refraction alignment on a refractiontime-distance (t-x) plot back to zero offset.

2,12 Overburden — The section above a refractor ora reflector.

2.13 Reciprocal Time — The common travel timeon reversed refraction profiles. Surface-to-surface timefrom a shotpoint at J to a geophone at B must equalthat from a shotpoint at B to a geophone at A,

2,14 Refraction — The change in direction of aseismic ray upon passing from a rarer to a densermedium or vice-versa with a different velocity.

2.15 Refraction Wave—A wave whichtravelsobliquely .downward from a source to a high-velocity formation(or marker), then within the formation, and finally,obliquely upward to detectors. The angles of incidenceand of emergence at the marker are critical angles.

2,16 Seismic Survey — A programme for mappinggeologic structure by creating seismic waves andobserving arrival time of the waves reflected fromacoustic-impedance contrasts or refracted through high-velocity members.

IS 15681:2006

2.17 Seismic Waves — Seismic waves are elasticwaves. Energy may be transmitted through the body ofan elastic solid by P-waves or S-waves or alongboundaries between media of different elastic propertiesby surface waves.

2.18 Seismograph — A recording system for seismicwaves.

2.19 Shear Modulus — The stress-strain ratio forsimple shear.

2.20 Shear Wave (S-wave) — A body wave in whichthe particle motion is perpendicular to the direction ofpropagation. Also called S-wave.

2.21 Shot/Hammer — To make an impact on ground,or tire an explosive to generate a seismic wave.

2.22 Snell’s Law — When a wave crosses a boundarybetween two isotropic medi%the wave changes directionso that the sine of the angle of incidence (angle betweenthe wavefront and a tangent to the boundary) divided bythe velocity in the first medium equals the sine of theangle of reii-action divided by the velocity in the secondmedium. Snell’s law applies to both P-and S-waves.

2.23 Surface Wave — Energy which travels along ornear the surface.

2.24 Time-Distance Curve — A plot of the arrivallime against the shotpoint-to-geophone distance. Alsocalled a r-x curve,used in interpreting refracted waves.The slopes of segments of the curve give the reciprocalsof the apparent velocities for various refractor beds.

2.25 Time Break — The mark on a seismic recordwhich indicates the shot instant or the time at whichthe seismic wave was generated.

2.26 Travel Time — The time between time break andthe recording of a seismic event.

2.27 Velocity— A vector quantity which indicates timerate of change of displacement, usually refers to thepropagation rate of a seismic wave without implyingany direction.

3 MEASURED PARAMETER ANDREPRESENTATIVE VALUES

The seismic refraction method gives the velocity ofcompressional (P-) waves in subsurface materials.Although the P-wave velocity can be a good indicatorof the type of soil or rock, it is not a unique indicator.Table 1 shows that each type of sediment or rock has awide range of seismic velocities, and many of theseranges overlap. While the seismic refraction techniquemeasures the seismic velocity of seismic waves in earthmaterials, it is the interpreter who, based on knowledgeof the local conditions or other data, or both, mustinterpret the seismic refraction data and arrive at ageologically reasonable solution.

P-wave velocities are generally greater for:

a) Denser rocks than lighter rocks;

b) Older rocks than younger rocks;

c) Igneous rocks than sedimentary rocks;

d) Solid rocks than rocks with cracks or tlactures;

e) Unweathered rocks than weathered rocks;

f) Consolidated sediments than unconsolidatedsediments;

g) Water-saturated unconsolidated sedimentsthan dry unconsolidated sediments; and

h) Wet soils than dry soils.

4 PURPOSE OF SEISMIC REFRACITON SURVEY

The seismic refraction survey has application in avariety of geological exploration problems, whereinformation on the depth and strength of subsurface

Table 1 Range of Velocities for Compressional Waves in Soil and Rock

(Clause 3)

S1No. NaturalSoiland Rock ftls mls

(1) (2) (3) (4)

i) Weathered surface material 800 to 2000 240t0610ii) Gravel or dry sand 1500 to 3600 460t0915

iii) Sand(saturated) 4 000to6000 1220 to 1830iv) Clay (saturated) 3000 to 9000 915 to 2750v) Water 4700 to 5500 I 430 to 1665

vi) Sea water 4800 to 5000 1460 to 1525vii) Sandstone 6000 to 13000 1 830to3960

viii) Shale 9000 to 14000 2 750to4270ix) Chalk 6000 to 13000 1830 to 3960@ Limestone 7000 to 20000 2 135t06 100

xi) Granite ‘ 15 Oooto 19000 4575 to 5800xii) Basalt 2ooooto 21100 6 000to6400

xiii) Quartizite/Phyllitic quartzite 13 120to 19680 4000 to 6000xiv) Quartizite phyllitic/phyllite 8200 to 11480 2500 to 3500xv) Gneiss 13 120to 19680 4000 to 6000

2

materials is required. These surveys provide subsurfaceinformation over large areas at relatively low cost,locate critical areas for more detailed testing by drillingand can readily eliminate less favorable alternativesites. Seismic surveys can also reduce the number ofboreholes required to test a particular site and improvecorrelation between boreholes. The following are theareas, where this method can be used to obtaininformation:

a)

b)

c)

d)

Problems of engineering geology

1)

2)

3)

4)

5)

6)

Depth to bedrock (thickness of overburdenandfor weathered rock).

Strength of the bedrock (looking forweak zones like fractures, shears andweatheringand for faults etc) for foundationstudies.

Rippability/excavability studies forquarries and dynamic elasticitydetermination.

Location of sink holes and other manmade objects.

Correlation of geological units betweenboreholes,

Monitoring of land slides.

Problems in alluvial prospecting

1) Indirect search for alluvial deposits.

2) Location of ancient stream channels.

3) Location of thick gravel beds.

Problems in hydrogeology

1) Thickness of aquifer overlyingimpermeable bedrock.

2) Detection of water table, mainly inalluvial aquifers.

3) Location of leakage zones.

Complimenta~ method as a constraint on theambiguities inherent in other geophysicalmethods.

5 METHODOLOGY

5.1 Measurement of subsurface conditions by theseismic refraction method requires a seismic energysource, trigger cable, geophones, geophone cable, anda seismograph.

The geophones and the seismic source must be placedin firm contact with the soil or rock. The geophonesare usually located in a line, sometimes referred to as ageophone spread. The seismic source maybe a sledgehammer, a mechanical device that strikes the ground,or some other type of impulse source. Explosives areused for deeper refractors or special conditions thatrequire greater energy. Geophones convert the ground

IS 15681:2006

vibrations into an electrical signal. This electrical signalis recorded and processed by the seismograph. Thetravel time of the seismic wave (from the source togeophone) is determined from the seismic wave form.

5.2 The seismic energy source generates elastic waveswhich travel through the soil or rock or both. When theseismic wave reaches the interface between twomaterials of different seismic velocities, the waves arerefracted according to Snell’s law. When the velocityVzis greater than Vi the ray will bend away from thenormal to the inter~ace on refraction, as shown inFig. 1A. In such a case for a particular angle ofincidence, known u the critical angle iC,the angle ofrefraction will be 90°. This gives rise to a criticallyreffacted ray that will then be traveling within the lowerniedium at the velocity Vz,at grazing incidence alongthe interface. It should be,noted that critical refractioncan only occur, where there is an increase in velocityat deeper refracting layer.

5.3 In Fig. lB, the position of a wavefront in the lowermedium is shown, together with the associatedwavefront being directed back into the overlying layer.The latter is known as a head wave. The ray paths arealso shown. The head wave is attached to the fastertraveling wavefiont in the deeper, higher velocitymedium of the refractor. The position of the wavefiontof the head wave may also be constructed as the envelopeof the secondary wavelets, by the application ofHuygen’s principle. Since the angle of critically reflactedray in the lower medium is 90° for rays, which are eitherentering or leaving the Vzrefracting layer, the head wavealso re-enters the overlying VI layer at the critical angle.Since the critically refracted waves are returned to thesurface in this manner, they are recorded.

5.4 A number of elastic waves are produced by aseismic energy source. As the compressional (P) wavehas the highest seismic velocity, it is the first wave toarrive at each geophone.

The P-wave velocity VPis dependent upon the bulkmodulus and the density and is given by

..(1)

where

VP= compressional wave velocity,

K = bulk modulus,

p = shear modulus, and

r = density.

The energy from the seismic source at each geophoneis recorded by the seismograph. From the positions ofthe pick-up points, the travel times are measured forthe first arrivals of seismic waves. The travel times arethen plotted at appropriate geophone distances on graphcalled a travel-time curve, or a time-distance curve, or

3

IS 15681: 2006

Source ~eophone

il

1A Ray Paths for a Critically Refracted Wave

sCRIWAL CROSS ova?DISTANCE DISTANCE G

1 x 1

Iz

r 4B REFRACTED

/

ci?AY v~7 VI

1B Wave Paths Associated with Critically Refracted Wave

FIG. 1 RAY ANDWAVE PATHSFORA CRITICALLYREFRACTEDWAVE

often abbreviated as t-x curve (see Fig. 2). Alignmentsof points on a t-x curve indicates the velocities ofseismic waves through different layers and providesthe information needed to calculate layer thicknesses.

5.5 Two Layers Analysis

Two layers with plane parallel boundaries are illustratedin Fig. 2. The first few arrival times are those of directarrivals through the first layer and the slope of the timedistance curve through those points, At/Ax, is simplythe reciprocal of the velocity of that layer; that is I/V’l,The energy that arrives at the detectors beyond thecrossover distance will plot along a line with slope ofl/V2,The one through these refracted arrivals will passthrough a projection on time axis to intersect it at atime called the intercept time ‘ti’.

Thickness of the layer at shotpoint from the intercepttime analysis is given by:

.(2)

True depth to the second layer is determined simply byadding half the shot depth to the value of 21computedby equation (2).

5.6 Three Layers Analysis

The geometry of ray path in the case of critical

refraction at the second interface is shown in Fig. 3.The seismic velocities of the three layers are Vj>V2>V1.The ray corresponding to the least travel time makes

v-1 Ian angle ‘i3 = sin ~ with the vertical in the uppermost

3

_, ~layer and angle ’23 = ‘in ~ with the vertical in the

3

second layer, iz~being the critical angle for the lowerinterface.

The thickness of 22 can be computed by:

‘2=4’i2-2z’~lf‘-(3)The depth to the lower interface is sum of Zzand 21where 21 is computed by the two-media formula(Equation 2) using the slopes of the first two segmentsof the time-distance curve and the intercept of thesecond segment.

5.7 Multilayer Analysis

If a structure has n horizontal layers with the thicknessz,, Z2, Z3, . . .... Zn and wave velocities V,, Vz, Vq,.....Vnresting on deeper material in which the wave velocityis Vn+la travel time with n+ 1 straight line segments isexpec~ed. The intercept time determined from wavereaching the deepest reflactor is given by:

4

IS 15681:2006

t 12

tlo

----------- ------- ------ -------------- ------- ------ ------ --------- ------ ------ ----- --------- ------ ------ --

t--------- --------- --11,1 I------- ------

------- -------

------- --- 1,,,1,811,1

-------- --- ;, ,1,,

/-.&-

X2 X4 X6 X8 Xlo X12SOURCE- RECEIVER DISTANCE (X)

V2

FIG. 2 TIME-DISTANCECURVEANDIUY PATHSFORA Two LAYERCASE

ti2

tit

x

FIG. 3 TIME-DISTANCECURVEANDGEOMETRYOFRAY PATHFORCRITICALREFRACTION

FORTHREELAYERCASE

5

9

IS 15681:2006

“ ZkTO= 2~Fcos ik[.+l) ...(4)

k-l k

and the thickness of the deepest layer by

[ 1Zn=+-~~cOsik(n+l)cost.+,). ..(5).

5.8 Dipping Layers

If the boundaries between interface are not parallel(that is, if they are dipping interfaces), the t-x curvewill give only apparent velocities for the refractinglayers usage of which can give erroneous depths.Reverse shooting field procedure can providecomplete protection against above said errors.Reverse shooting means firing a shot at both ends ofthe seismic line so that arrival times at each detectorare measured ftom both directions. The case of adipping boundary and its effect on travel-time plotsis as shown in Fig. 4.

The condition of reciprocity provides a valuableconstraint. The condition of reciprocity states that thetotal travel time from A to D must be equal to the totaltravel-time from D to A.

The gradients of the travel-time curves of refkactedarrivals along the forward and reverse profile lines yieldthe downdip ( VJ and up dip (VJ apparent velocitirespectively (see Fig, 4).

ttm

tj

t

From the forward direction:

. ..(6)

And, from the reverse direction

k’2”= ~sin (12- e) . ..(7)

Solving for ilz and e

ilz = l/2[sin-l(Vl/Vu)] + [sin-l(V1/Vzu)] . ..(8)

e = l/2[sin-l(VJVU)]’- [sin-l(V/V2u)] ., .(9) %+

The perpendiculardistances Z and Z’ to the interfaceunder the two en& of the profile are obtained fkomtheintercept times ?,andti’the travel-time curves obtained .

in the forwardand reverse direction.

Z’..-K-2COS42

. ..(10)

. ..(11)

By using the computed refractor dip e, theperpendicular depths Z and Z’ can be converted intovertical depths h and h’ using

h = Zicos e . ..(12)

FIG. 4 TIME-DISTANCECURVEANDGEOMETRYOFRAY PATHFORCRITICALREFRACTIONFORTwo LAYERDIPPINGCASE

6

1S 15681:2006

6 EQUIPMENTS

Equipments used for surface seismic refractionmeasurement include a seismograph, geophones,geophone cable, an energy source and a trigger cableor radio link. A wide variety of seismic geophysicalequipment is available and the choice of equipment forrefraction survey should be made to ineet or exceedthe objectives of the survey.

6.1 Seismographs

A wide variety of seismographs are available fromdifferent manufacturers. They range from relativelysimple, single-channel units to very sophisticatedmultichannel units. Most engineering seismographssample, record and display the seismic wave digitally.

6.1.1 Single Channel Seismograph

A single channel seismograph is the simplest seismidrefraction instrument and is used with a singlegeophone. The geophone is usually placed at a fixedloGation and the ground is struck with a hammer atincreasing distances from the geophone. Seismic wavearrival times are identified on the instrument. For simplegeologic conditions and small projects, a single channelunit is satisfactory,

6.1.2 Multiple Channel Seismograph

Multichannel seismographs use 6, 12,24,48 or more,geophones. With a multichannel seismograph, theseismic wave forms are recorded simultaneously forall geophones.

6.1.3 Signal Enhancement

Signal enhancement or energy stacking that improvesthe signal to noise ratio is available in mostseismographs. Signal enhancement is accomplished byadding the refi-acted seismic signals for a number ofimpacts. This process increases the signal to noise ratioby summing the amplitude of the coherent seismicsignals while reducing the amplitude of the randomnoise by averaging.

6.2 Geophone and Cable

A geophone transforms the P-wave energy into a voltagethat can be recorded by the seismograph. For refractionwork, the natural frequency of the geophones variesfrom 4 Hz to 14 Hz and these geophones have a flatfrequency response between 4 Hz to 14 Hz. The signalsffom geophones is brought to the seismograph throughgeophone cable.

6.3 Hydrophores

Hydrophore is a detector which is sensitive to variationsin pressure. The sensing element is usually apiezoelectric ceramic material, such as, barium titanate,

lead zirconate, or lead metaniobate. Piezoelectrichydrophores are high-impedance devices and signalsfrom hydrophores, or hydrophore arrays, may bepassed through pre-amplifiers or impedance-matchingtransformers before transmission through the streamerto the recording instruments.

6.4 Energy Sources

The selection of seismic refraction energy sources isdependent upon the depth of investigation and geologicconditions. Four types of energy sources are commonlyused in seismic refraction surveys; sledge hammers,mechanical weight drop or impact devices, projectile

(w) sowces and explosives.

7 PLANNING THE SURVEY

7.1 Planning and design of a seismic refraction surveyshould be done with due consideration of the objectivesof the survey tid the characteristics of the site. Thesefactors determine the survey, design, the equipment tobe used, the level of effort, the interpretation methodselected, and budget necessary to achieve the desiredresults. Important considerations include site geology,depth of investigation, topography and access. Thepresence of noise-generating activities and operationalconstraints, should also be considered. It is a goodpractice to obtain as much relevant information (forexample, data from any previous seismic refractionwork, drilling, geologic and geophysical logs of thestudy area, topographic maps or aerial photos, or both)about the site, prior to planning a survey andmobilization to the site.

7.2 The first important consideration in planning a fieldrefraction survey is the spacing between the geophones.The spacing will depend on the desired depth ofexploration.. Selection of detector spacing is alsodetermined by the required detail of the refractorgeometry, This is because the sampling interval ofinterpretation points on the refractor is approximatelyequal to the detector spacing on the surface. Thus, thehorizontal resolution of the method for subsurfacetargets is equal to the detector spacing. Once the spacingbetween geophones is decided, the “spread” length thatis covered by one shot is fixed by the number ofchannels of the seismic equipment being deployed.Usually, in detailed engineering surveys, a 5 m intervalbetween geophones is appropriate. If the profile lengthrequired to get the desired subsurface information ismore than one “spread” length, the time-distanceinformation should be collected by overlapping as manyspreads as required. In doing so, at least two geophonesshould be kept overlapping between successive spreadsto tie the timings from different shots to make profilecontinuous. Usually, five to seven shots are requiredfor each spread. The chainages of shots are given below:

7

IS 15681:2006

a)

b)

c)

d)

Depth of investigation : uptol OmGeophone interval : 5mNumber of channels : 12

Spread Length Shots

O-55m –50 m, 2.5 m, 27.5 m,52.5 m, 77.5 m, 102.5 m

50-105 m 2.5 m, 27.5 m, 52.5 m,77.5 m, 102.5 m, 127.5 m,152.5 m ‘

100-155.5 m 52.5 m, 77.5 m, 102.5 m,127.5 m, 152.5 m, 177.5 m,202.5 m

150-205 m 102.5 m, 127.5 m, 152.5 m,177.5 m, 202.5 m, 255 m

Depth of investigation up to : 20mGeophone interval : 5mNumber of channels : 12

Spread Length Shots

O-55m –100 m, 2.5 m, 27.5 m,52.5 m, 77.5 m, 102.5 m,127.5 m, 152.5 m

50-105 m –50 m, 2.5 m, 27.5 m,52.5 m, 77.5 m 102.5 m,127.5 m, 152.5 m, 177.5 m,202.5 m

100-155m 2.5 m, 27.5 m, 52.5 m,77.5 m, 102.5 m, 127.5 m,152.5 m, 177.5 m, 202.5 m,227.5 m, 252.5 m

Depth of investigation up to : 10mGeophone interval : 5mNumber of channels : 24

Spread Length Shots

O-l15m –100 m, 2.5 m, 27.5 m,57.5 m, 87.5 m, 112.5 m,215 m

Depth of investigation up to : 20mGeophone interval : 10mNumber of channels : 24

Spread Length Shots

0-230 m –200m,5m,55m, l15m,175 m, 225 m, 430 m

7.3 The advantage of locating the shotpoints betweentwo geophones is that the reciprocal times betweendifferent shotpoints can be read directly instead ofextrapolating beyond the end geophone, which canintroduce errors in measurement of reciprocal time, incase of uneven refractor topography. Secondly, it doesnot disturb the ground under adjacent geophone stations.Any disturbance of the ground caused by placing a shot

too close to a geophone station will delay waves arrivingat the geophone for all subsequent recording, which willnot allow separation of this delay in arrival time fromthe increase in arrival time coming from deeper layers.

7.4 When explosives are used as sources, chargeweights adequate to produce sharp and well definedfirst arrivals on the seismic records are needed. Chargeweight is dependent on:

a) geology,

b) length of the seismic profile,

c) shot depth, and

d) amount ofbackground noise at the site of work.

From field experience at various sites in India, thefollowing charges are found to be sticient for providingclear first arrivals and are recommended for obtaininggood records. Generally, when top layer is not very loose,half a stick (130 glstick) of gelatine for central shot (thatis, shot in the middle of the profile) and one stick for theshotpoints at the end of spreads is adequate. For 50 mand 100 m offset shots, two and four sticks respectivelyare enough to produce clear first arrivals. Shot holedepthsmay vtuy from 1m to 3 m depending on the natureof top layer and difficulty of drilling the holes, the holesbeing deeper in case of loose top surface layer. Couplingof explosive energy with the ground should be improvedby burying charges deeper and saturating the shot holewith water, This will help in transmitting most of theseismic energy into the ground.

7.4.1 Special blasting caps (instantaneous blasting orzero delay type) should be used for seismic survey. Thegeophones should be held vertically and firmly coupledto the ground, to avoid any delay in the recording ofseismic wave arrivals.

7.4.2 It is desirable to maintain a constant shot depthfor the survey at a particular site. If it is not feasible tomaintain the same depth, a shot depth correction shouldbe added to all arrival times to effectively yield a traveltime, which would have been recorded had the shotbeen at the surface of the ground,

7.5 Seismic cables are manufactured with fixedspacing, that is, the take-out or polarized connector foreach detector is moulded into the cable and the spacingbetween these take-outs is fixed. It is desirable to haveat least two cables with spacings 15 m and 30 m. For amajority of the engineering surveys, where the samplinginterval is less than 15 m and the depth of explorationis seldom more than 100 m, a 15 m spacing cableshould be employed. H’owever, for investigationswhere marker of interest (refractor) is deeper, seismiccable with 30 m spacing should be used.

The cable shouId be laid along the ground in a straightline. If the terrain along the seismic line has any relief,

8

ground level survey should be carried out so that thesubsurface layer thicknesses can be plotted in trueelevation of the site.

7.6 In water, the picking up of the arrivals ofcompressional waves is done by hydrophores. Thehydrophore cable is towed beyond the ship and at someplaces the cable is tied with the buoy rope so that thecable will float on the water surface. The hydrophorecable is connected to a multichannel seismograph(see Fig. 5). Shots are fired on the bottom with fixedintervals.

8 LIMITATION OF SEISMIC REFRACTIONTECHNIQUE

A hidden layer, or blind layer, is one that is undetectableby refraction surveying. In practice, there are twodifferent types of hidden layer problem.

A layer may simply not give rise to first arrivals, thatis, rays traveling to deeper levels may arrive beforethose critically refi-actedat the top of the layer in question(see Fig. 6A). This may result ffom the thickness ofthe layer, or from the closeness of its velocity to thatof the overlying layer. In such a case, a method ofsurvey involving recognition of only first arrivals wilifail to detect the layer. The problem maybe overcomeby firing a shot in a deep hole so that arrivals from theintermediate layer are recorded at the surface.

A more insidious type of hidden layer problem isassociated with a low velocity layer, as illustrated inFig. 6B. Rays cannot be critically refracted at the top ofsuch a layer and the layer will, therefore, not give riseto head waves. Hence, a low velocity layer cannot bedetected by refraction surveying although the top of thelow velocity layer gives rise to wide angle reflectionsthat may be detected during a refraction survey.

Seismographinside the boat Hydrophore

/ Cable

In the presence of a low velocity layer, the interpretationof travel-time curves leads to an overestimation of thedepth to underlying interfaces. Low velocity layers area hazard in all types of refraction seismology. On asmall scale, a peat layer in muds and sands abovebedrock may escape detection, leading to a falseestimation of foundation conditions and rock depthsbeneath a construction site.

9 INTERPRETATION

9.1 In some limited cases, quantitative interpretationof the data may not be required and a simple qualitativeinterpretation may be sufficient. Examples of qualitativeand semi-quantitative interpretation may include thelateral location of a buried channel without a concernfor its depth or minimum depth to rock calculations. Inmost cases, however, a quantitative interpretation willbe necessary.

9.2 The level of effort involved in the interpretationwill depend upon the objectives of the survey and thedetail desired that, in turn, will determine the methodof interpretation. A number of manual methods andcomputer programmed are available for interpretation.While the solution for these methods can be carriedout manually, the process can be Iabour intensive forthe more sophisticated methods.

9.3 A problem inherent in all geophysical studies isthe non-unique correlation between possible geologicmodels and a single set of field data. This ambiguitycan be resolved only through the use of sufficientgeologic data and by an experienced interpreter.

9.4 The first step in the interpretation process is todetermine the time interval from the impact of theseismic source to the first arrival of energy at eachgeophone. When the first arrivals are sharp and there

Bouv

FIG. 5 LAYOUTOFHYDROPHORESFORUNDERWATERSEISMICREFRACTIONSURVEY

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t

x

v,> VI

6A Hidden Layer Problem in Seismic Refraction

t

\ /A

v, * VI

6B Blind Zone Problem in Seismic Refraction

FIG. 6 PROBLEMSINSEISMICREFRACTION

is no ambient noise, this procedure is straightforward.In many cases, noise in the data will make picking thefirst arrival times difficult. To minimize errors, aconsistent approach to the picking of the arrival timesshould be used. Care should be taken to ensure thateach trace is picked at the same point, that is, at thefirst point of movement or the point of maximumcurvature. This procedure will make the interpretationa more uniform process, as the data will be consistentfrom one trace to the next. In some cases, a first arrivalpick from one or more geophones may be uncertain.If this occurs, these picks should be noted. If acomputer programme is used to make first arrivalpicks, these picks should be checked by the individualdoing the processing and interpretation.

Corrections to travel-time for elevation or other

geometric factors are then made. The two main typesof corrections are elevation corrections and weatheringcorrections. Both are used to adjust field-derived traveltimes to some selected datum, so that straight-linesegments on the time distance plot can be associatedwith subsurface refractor. These corrections can beapplied manually or by computer.

With the corrected travel-time data, a time-distance plotof arrival times versus shotpoint-to-geophone distancecan be constructed. Lines are then fitted to these pointsto complete a time-distance plot. These time-distanceplots are the foundation of seismic refractioninterpretation.

The methods given in 9.4.1 to 9.4.3 are generally usedfor seismic refraction interpretation.

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9.4.1 Intercept 71meMethod (ITM)

The standard ITM is probably the best known of allthe methods for the interpretation of seismic refractiondata. It can be described as the rigorous application ofSnell’s law to a subsurface model consisting ofhomogeneous layers and plane interfaces. These planarinterfaces can be either horizontal or dipping. Theintercept time method requires that a constant seismicvelocity exists in the overburden and in the reffactorwithin a single geophone spread.

The ITM method can be applied where a limitednumber ofrefiactor depth determinations are requiredwithin a single geophone spread; the surface of therefractor can be satisfactorily approximated by a plan(horizontal or dipping); lateral variations in seismicvelocity of the subsurface layers can be neglected; andthin intermediate seismic velocity layers and seismicvelocity inversions can be neglected. This method isdescribed in 5.

9.4.2 Conventional Reciprocal Method (CRM)

In many ways, the conventional reciprocal method (orHawkin’smethod) can be considered to be an improvedITM, whereby the computations are extended from theshotpoints to each geophone location. As with the ITM,the analysis can be separated into the two distinct stagesof the determination of refractor velocities, and thecomputation of a depth related term, similar to half theintercept time, called the time-depth.

A two layer case is given in Fig. 7. The time depth t~atthe geophone station is obtained by adding together thetravel times from both shotpoints to the geophone station,subtracting the shotpoint-to-shotpoint travel-time (thereciprocal time), and halving the result. Thus the time-depth tGat the geophone station G is given by:

tG= l/2(tAo + tBG– tm) . ..(14)

*G,rB~and tA~are the travel times of the criticalwhere trays from A to G, Bto G, and A to B respectively.

Substituting travel times in terms of their segment traveltimes as distance times velocity

t~ = l/2(GX/Vl + GY/V1 –XY/V2) . ..(15)

If the refractor is assumed to be plane bemeen thepoints X and Y,then from the symmetry,

tG= (GX/V1 – PX/ VJ . ..(16)

which is the expression for the time-depth. “

The depth to the important refi-actor at a geophonestation G, Z~, is calculated tlom the equation,

ZG= tG~ . ..(17)

where tG is the time-depth to the important refractor

and ~~ is the corresponding composite depthconversion factor.

The composite depth conversion factor is calculatedby summing the thicknesses (ZJ upto the importantrefractor and dividing this depth by the time-depth orhalf intercept time to the important refractor ~n). Thus,the composite depth conversion factor, V, to nthrefractor is

..(18)

This method can be applied where depths to therefractor are required at each geophone; the surface ofthe refractor has some reliefi lateral variations inseismic velocity of the subsurface layers (over thelength of the spread) can be neglected; and thinintermediate seismic velocity layers and seismicvelocity inversions can be neglected.

WV’FIG. 7 RAY PATHSCONSIDEREDFORCRM

METHODOFINTERPRETATION

9.4.3 Generalized Reciprocal Method (GRM)

The generalized reciprocal method, popularly known asGRM, is the most efficient and accurate interpretationtechnique for seismic reflection data. The GRM candefine layers with varying thicknesses and seismicvelocities, unlike the conventional intercept time method.

Like the conventional reciprocal method, the GRM usesboth forward and reverse arrival times, and is relativelyinsensitive to dip angles up to about 20°. As a result,depth calculations to an undulating refractor areparticularly convenient, even when the overlying stratahave velocity gradients.

The conventional reciprocal method smoothensrefractor irregularities because it assumes a planerefractor between the points of emergence of theforward and reverse rays. The GRM employs theprinciple of migration. Arrival times at two geophones,separated by what is termed the XY-distance, are usedin refractor velocity analysis and time-depthcalculations. A range of XY spacings is used, and theoptimum value is selected using various tests associatedwith the method. At the optimum XY spacing, theforward and reverse rays emerge from near the samepoint on the refractor, so the refractor need only beplane over a very small interval.

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The two major steps involved in using GRM are asfollows:

a) Velocity analysis function:

The velocity analysis Ilmction, tv, is given by

tv = (tAy– tBx+ tM)/2 . ..(19)

The locations of points A, B, X and Y areillustrated in Fig. 8. Each tvvalue is referencedto the point G, which is half-way between theforward direction emerging point Yand reversedirection emerging point X. The velocityanalysis fi.mction tv is dependent on the XY-distance. Velocity analysis function fordifferent XY values is calculated and velocityanalysis curves for different XY values areplotted. The change of lateral velocity alongthe refractor can be readily identified fromthese curves. The velocity analysis functioncurve which exihibits the least scattering of datapoints provides the optimum XYvalue and thebest fit line to this curve yields the true refractor

velocity. This optimum XY value is used forsubsurface layer depth calculation.

b) Time-depth function:

The time-depth, t~ is given by

‘G = [tAy+ ‘Bx– (tAB+ xY/ vn)]/2 . ..(20)

where J’. is the true velocity of the refractorat each geophone position as determined fromvelocity analysis fimction.

The time-depth, which is also plotted with respect toG, is a measure of the depth to the refractor, expressed

in units of time it is related to the thicknesses ZjGof thelayers by:

n-l

tG= ~z,G(vn* - lfy /vnvj . ..(21)j-l

A set of curves for variousXYvalues are obtained. Thecurve that yields the maximum detail, that is therefi-actinghorizon in time corresponds to the optimumXY value. The optimum XY value obtained from boththe velocity analysis and the timedepth fimctions shouldbe the same, whioh is used for final data interpretation.

The GRM is applied where lateral variations in seismicvelocity within a single geophone spread, thinintermediate seismic velocity layers, and seismicvelocity inversions cannot be neglected. Geophonespacing for this method is generally smaller to providesutllcient spatial data, The GRh4 requires the greatestlevel of field and interpretation effort.

9.5 The choice of interpretation method may vary fromsite to site and will depend upon the detail requiredfrom the seismic refraction survey and the complexityof the geology at the site. The interpretation methodwill, in turn, determine the approach and level of effortrequired in the field.

10 PRESENTATION OF DATA

The final seismic refraction interpretation is representedas a depth section, a contour map, or other drawingsthat illustrate the general geologic and hydrogeologicconditions and any anomalous conditions at a site.Figure 9 shows the typical travel time curves andcorresponding depth section.

A DATUM x 6 Y B

‘Bfl I L tAY //\ n I -. II

\\VI \ Lz,ta/

v~

FIG. 8 RAY PATHSCONSIDEREDFORGRM I@THOD OFINTERPRETATION

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70

60

50

.~ 40mx

10

0

5

0-?

zoG -s3w-1Id

OtSTANCE (m)

i

-J 1 I I i I [ I I [ I I I t 1 I I n 1 I I 1 I

o so 100 150 200 220DISTANCE (m)

FIG. 9 TYPICALTRAVEL-TIMECURVESANDCORRESPONDINGDEPTHSECTION

FORTwo LAYER CASE

70

“60

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“ 40

-30

“20

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-15

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